The present invention relates to a semiconductor laser device capable of emitting a plurality of laser beams of different wavelengths, and a method for fabricating the same.
In recent years, there is an increasing demand for semiconductor laser devices in many industrial fields, and active researches and development have been performed primarily for semiconductor laser devices including a group III-V compound semiconductor layer, particularly a compound semiconductor layer containing GaAs or InP.
In the field of optical information processing, systems have been realized in which data is written or read with a semiconductor laser device which includes an AlGaAs layer and oscillates an infrared laser beam having a wavelength in a 780 nm band. Such semiconductor laser devices have already been widely used in the field of compact disks (CDs), etc.
A recording apparatus for use with a medium such as an magneto-optical disk having a greater capacity than that of a CD employs a semiconductor laser device which includes an AlGaInP layer and oscillates a laser beam in a 680 nm band shorter than a 780 nm band.
Recently, a digital video disk (DVD) system capable of long-time playback of high-definition images requires a semiconductor laser device which emits a red laser beam having a wavelength in a 650 nm band. Thus, it has been attempted to improve the recording density of an optical disk through the reduction in oscillation wavelength.
Some DVD apparatuses for reading DVD data have compatibility with CDs which allows for reading data of both DVDs and CDs so that one can utilize conventional CD data as well as DVD data. Therefore, the light source of the pickup head device of such a DVD apparatus needs to be provided with two semiconductor laser devices, including a first semiconductor laser device which includes an AlGaAs layer and emits an infrared laser beam in a 780 nm band, and a second semiconductor laser device which includes an AlGaInP layer and emits a red laser beam in a 650 nm band.
In such a case, if an optical processing system is provided for each of the semiconductor laser devices, it is necessary to provide an optical system for combining the laser beam in a 780 nm band with the laser beam in a 650 nm band, thereby complicating the structure of the pickup head device and imposing a limit on the downsizing of the pickup head device.
In view of this, a hybrid type semiconductor laser device in which two semiconductor laser devices are arranged adjacent to each other, or a monolithic type semiconductor laser device in which two semiconductor laminated structures are provided in parallel to each other on a single substrate has been proposed in the art (see Japanese Laid-Open Patent Publication No. 11-186651 and Proc. of the 60th Fall Meeting of JSAP, 3a-ZC-10).
FIG. 21 illustrates an example of a conventional monolithic type semiconductor laser device. The semiconductor laser device includes a first semiconductor laminated structure 2 including an AlGaInP layer and a second semiconductor laminated structure 3 including an AlGaAs layer which are provided on a single semiconductor substrate 1, emitting a laser beam in a 650 nm band from a light-emitting spot 4 of the first semiconductor laminated structure 2 and emitting a laser beam in a 780 nm band from a light-emitting spot 5 of the second semiconductor laminated structure 3.
With hybrid type or monolithic type semiconductor laser device as described above, it is no longer necessary to provide an optical system for combining two laser beams of different wavelengths, thereby allowing for simplification and downsizing of the pickup head device.
However, in the hybrid type semiconductor laser device, the pitch of the two semiconductor laser devices is influenced by the width dimension of each semiconductor laser device. As a result, the pitch of the light-emitting spots is on the order of 100 μm or more.
In the monolithic type semiconductor laser device, it is necessary to provide two semiconductor laminated structures on a single semiconductor substrate. As a result, the pitch of the light-emitting spots is on the order of 10 nm or more due to the limit of the process for separating the two semiconductor laminated structures from each other.
An optical pickup head device needs to have a half mirror for directing an emitted laser beam toward the optical disk, an object lens for focusing the laser beam having passed through the half mirror into a spot on the optical disk, a photodetector for detecting the laser beam reflected from the optical disk, etc.
However, since the object lens has been downsized along with the downsizing of the optical pickup head device, the focusing characteristic of the object lens varies due to the variation in the laser beam incident point on the object lens (the position on the object lens where the laser beam is incident varies because there are two different light-emitting spots). As a result, it is difficult to focus the laser beam having passed through the object lens into a tiny spot on the optical disk.
Moreover, when the angle at which the laser beam having passed through the object lens is incident upon the optical disk varies, the direction in which the laser beam is reflected from the optical disk also varies, thereby making it necessary to provide two photodetectors.